As described in the '101 application, an on-going objective of the telecommunications industry is to increase the number of customers that may be serviced by a single channel bank connection with a network, such as but not limited to a DS3 or OC3 network connection. In the past, this objective has been dealt with primarily by using either bus extension or channel bank subtending. According to the bus extension approach, the physical length of the backplane bus of a master channel bank is increased by means of a bus extension (e.g., ribbon) cable, in order to allow more line cards to be daisy chain-connected to the bus. In this type of architecture, upstream directed data (from the customer to the network) passes from a customer interface with a line card onto the bus extension, and then into the switching fabric through which a connection with the network is afforded, using a policing engine (a flow control mechanism) resident within the switch fabric of the master channel bank. Downstream directed data (from the network to a customer) enters the switch fabric where it is scheduled for downstream routing, and then transported across the bus extension into a line card and passed on to the customer. Information concerning policing, scheduling and queuing engines is contained in ITU-T Recommendation I-371 Traffic Control and Congestion Control for B-ISDN.
In the second approach, multiple line card slots of the master channel bank are usurped by channel bank expansion cards, which are respectively linked to associated subtended channel banks. In such an architecture, upstream-directed data passes from the customer interface into a line card of one of the subtended channel banks. From the line card, the data passes into the subtended channel bank's switch fabric, where the data is policed and scheduled for delivery to the network, via the master channel bank's network connection. However, before it is delivered to the network, the data is passed over the primary channel bank's bus into another switch fabric, where is again policed and scheduled. It is then passed onto the network connection for delivery to the network. Downstream-directed data enters the switch fabric of the master channel bank from the network connection, and is transferred therefrom down to a network card (which typically occupies two line card slots of the master channel bank), which passes the data on to the switch fabric of a subtended channel bank, for delivery to a line card of that channel bank. Every time data enters a subtended bank's switch fabric it is policed and scheduled.
The bus extension approach has a number of limitations. For example, given the fact that the extension bus is shared among the master channel bank and one or more expansion channel banks, a problem may arise if one of the channel bank cards malfunctions and seizes control of the extension bus. This could terminate or prevent data transfers on the shared bus. Moreover, since the channel banks are connected by way of a bus extension cable, there is a practical, noise-coupling limitation as to the number of channel banks that can be daisy chained together. In addition, the bus extension cable is usually bulky and expensive.
The network card extension approach is also limited by a number of factors. One is the fact that since the channel banks are interlinked by using network connections that usurp multiple line card slots, the primary bank loses the availability of line cards that would otherwise be used to provide service to customers. Also, the use of network cards adds a greater expense for data expansion, and limits the number of customers that can be serviced by the host channel bank. In addition, as each of the subtended channel banks requires a network connection, the switch fabric must be replicated on each subtended channel bank, which implies the need to replicate policing and scheduling mechanisms, and adds to the expense.
In accordance with the invention disclosed in the '101 application, these and other drawbacks of conventional channel bank expansion architectures are effectively obviated by a single switch fabric-based, multi-channel bank architecture, that contains a primary channel bank or master shelf, and one or more expansion channel banks or subtended expansion shelves, which are linked with the primary channel bank by way of upstream and downstream communication links, and distributed among which channel banks are respective policing engines and cell rate control mechanisms for controlling upstream-directed communications from customer ports to the network.